It is well known that cigarette smoke contains many harmful substances. Consequently, there exists an interest in the industry to produce cigarettes the smoke from which contains considerably fewer harmful substances. There are various approaches to reducing the amount of such substances. For example, cigarettes are often equipped with filters, typically made of cellulose acetate, which can absorb a part of the particulate phase of the smoke, generally called “tar”. Other methods aim to dilute the smoke generated in the cigarette, for example with an air flow flowing through a perforation in the tipping paper. In addition, by its defined air permeability, the cigarette paper wrapping the tobacco rod allows air to flow into the tobacco rod of the cigarette, which dilutes the smoke. Finally, the amount of harmful substances in the smoke of a cigarette can be influenced by selection of the tobacco blend.
Typical cigarette papers comprise cellulose fibers, among others, which are produced from wood, flax or other materials. In addition, mixtures of cellulose fibers of different origins are used.
A characteristic property of the cigarette paper of great technical importance is its air permeability. It describes the permeability of the paper to an air-flow, which is caused by a pressure difference between the two sides of the paper. More precisely, it describes the volume of air flowing through the paper per unit time, per unit area and per pressure difference and thus has the dimension cm3/(min cm2 kPa), which is frequently called the CORESTA Unit (CU), wherein 1 CU=1 cm3/(min cm2 kPa). Known cigarette papers exhibit an air permeability between 10 CU and 300 CU, wherein the range of 20 CU to 120 CU is most commonly used.
The air permeability can, for example, be determined in accordance with ISO 2965. According to ISO 2965, the volume of air flowing through a rectangular opening with a width of 10 mm and a length of 20 mm per unit time at a pressure difference of 1 kPa is determined and expressed in CU units. Alternatively, in accordance with ISO 2965, a rectangular opening with a width of 2 mm and a length of 15 mm can be used.
An assumption which very closely approximates to conventional cigarette papers is that the air-flow through the cigarette paper is proportional to the pressure difference in the range of pressure differences to which the cigarette paper on a cigarette during smoking is exposed. Hence a linear relationship exists between pressure difference and air-flow through the paper. The typical pressure difference between the inside and the outside of the cigarette during smoking is between 0 kPa and 1.0 kPa.
In Annex D.2 (ISO 2965:2009), ISO 2965 allows the non-linearity of the relationship between air-flow and pressure difference to be estimated. For this purpose, at least one measurement of the air-flow Q1 at a pressure difference p1=1.0 kPa and one measurement of the air-flow Q2 at a pressure difference of p2=0.25 kPa is made. The exponent k is calculated from both measured values in accordance with equation (D.6) in ISO 2965:2009 by
  k  =            log      ⁢                        Q          1                          Q          2                            log      ⁢                        p          1                          p          2                    
This exponent k describes the non-linearity and is in the range of 0.5 to 1.0, whereby a value of 1.0 describes linear relationships. In this regard, conventional cigarette paper, as stated before, exhibits linear behavior and thus has a value for the exponent k of between 0.98 and 1.0.
Measurement devices that measure the air permeability in accordance with ISO 2965 are commercially available and in most cases also allow the exponent k to be determined. Consequently, here, whenever reference to a value of the exponent k is made, it should be understood that the value is calculated from a measurement at 1.0 kPa and a measurement at 0.25 kPa in accordance with ISO 2965, Annex D.2, with a measuring head with a rectangular opening of 2 mm by 15 mm.
Further technical requirements for the cigarette paper are related to the processability of the cigarette paper on the cigarette machine, for example basis weight, thickness, elongation at break and tensile strength. Additionally, there are further requirements related to the optical properties of the cigarette paper, for example, opacity and whiteness. Furthermore, there are extensive legal regulations for the ingredients allowed in cigarette paper.
But above all, for cigarette paper the influence on the taste of the cigarette plays a major role, as the cigarette is burnt together with the tobacco and the combustion products from the cigarette paper form part of the smoke. Hence, it is important that all modifications to the cigarette paper leave the cigarette paper chemically in an as natural state as possible, so that the components of the cigarette paper do not have a negative influence on the taste of the smoke.
At the same time, there is an interest in controlling the smoke yields of a cigarette by modification of the cigarette paper.